The invention provides for oxidatively resistant carbon/carbon (C/C) composites and other graphite-like material, a novel coating combination for effecting increased oxidation resistance of the carbon/carbon composites and other graphite-like material, a method for the preparation of these materials, and their use in high temperature applications, preferably in brakes on aircraft.
When the carbon/carbon composites are utilized as a stack of discs on aircraft brakes, they are required to absorb large amounts of kinetic energy in order to stop the aircraft during landing or in the event of a rejected take-off. During some of the stops, the carbon is heated to sufficiently high temperatures that surfaces exposed to air will oxidize. Some conventional carbon composites have the necessary thermal and mechanical properties required for specific brake designs; however, these conventional composites have open porosities (typically 5% to 10%) which permit internal oxidation. The internal oxidation weakens the material in and around the brake rotor lugs or stator slots, which are areas that transmit the torque during braking.
Damage associated with oxidation has led to premature removal of carbon brake discs on a variety of aircraft, from all current brake manufacturers. Thus, the overall objectives of the invention are to protect carbon/carbon composites or graphites from oxidation at elevated temperatures. Both field data and theoretical models indicate that modern carbon/carbon aircraft brakes used in the transportation industry frequently see peak temperatures above 1500xc2x0 F. and that some models (including brakes for use in military aircraft or freight hauling) routinely experience extended periods between 1450xc2x0 F. to 2200xc2x0 F. over their service lives.
In order to inhibit the oxidation of carbon/carbon composite articles, phosphoric acid based penetrants have been used extensively. These penetrants are disclosed in McAllister et al., U.S. Pat. No. 4,837,073 and Stover et al., U.S. Pat. No. 5,759,622.
McAllister et al. teach an aqueous penetrant composition containing water, phosphoric acid, MnHPO4 1.6H2O, AlH2PO4, 50% B2O3, and Zn3PO4. McAllister et al also teach that a barrier coating of silicon carbide can be formed on the C/C composite prior to applying the penetrant composition.
A disadvantage of the coated composite of McAllister et al. is that the overall coating thickness is relatively high. When the thickness is too high, there is an undesirable increase in mass and the loss of the overall brake dimensional tolerance. The combination of the barrier coating and the penetrant have a thickness of 125-250 microns (5-10 mm). Thick coatings such as these are susceptible to spallation on low expansions typical of C/C composites.
Stover et al. teach an aqueous penetrant composition which comprises (a) phosphoric acid, (b)(i) a metal phosphate or (ii) a combination of a zinc salt and an aluminum salt, and (c) a compatible wetting agent selected from the group consisting of polyols, alkoxylated monohydric alcohols, silicone surfactant and mixtures thereof. Stover et al. also teach that a barrier coating can be formed on the C/C composite prior to applying the penetrant composition. These barrier coatings include silicon carbide, titanium carbide, boron carbide and silicon oxycarbide.
A disadvantage to both the penetrant type systems of Stover et al. and McAllister et al. results from the fact that an inert atmosphere must be used when heat curing the penetrant in the composite. During this heat curing step, the phosphoric acid diffuses throughout the internal porosity of the composite. The diffused phosphoric acid is partially reduced to elemental phosphorus and lower phosphoric oxides wherever the phosphoric acid comes in contact with carbon. During use of these composites, the phosphorus vaporizes, is forced to the surface and burns on contact with the air. Also, the phosphoric acid and other coating ingredients can be transported to the wearing surface where they can degrade the brake""s friction and wear properties.
Through the use of a novel dual coating system, which includes a coat prepared from a glass frit slurry, the composite of the present invention has a significantly improved oxidative resistance at the high end of the typical operating temperature range over the coatings known in the art.
Although, carbon/carbon composites and other carbon materials, such as graphite, rank among the most inert and least reactive materials known at high temperatures, oxidation is a highly significant cause of deterioration of strength and loss of material. Thus, retardation of the oxidation reactions could be highly beneficial in lowering consumption, both by direct oxidation and by lessening breakage caused by oxidation-induced loss of strength.
Accordingly, the present invention, in part, provides a dual coating for effecting an oxidation inhibiting barrier to graphite and similar carbonaceous bodies.
The present invention also provides, in part, a glass frit slurry that can be applied to carbon/carbon composites and graphite-like materials to achieve an oxidation prevention coating.
The present invention also provides, in part, a method for producing the glass frit slurry to be applied to carbon/carbon composites and graphite-like materials to produce an oxidation prevention coating.
The present invention also provides, in part, a method for applying the glass frit slurry to the carbon/carbon composites and graphite-like materials to produce an oxidation prevention coating.
The oxidatively resistant surface coated carbon/carbon composites and graphite-like material according to the present invention are preferably used in brakes on aircraft, but may also be used in other high temperature applications, such as electrodes for arc melting of steel, mold stock for metal casting, rocket nozzles, furnace linings, and Hall cell anodes.
In particular, the present invention also provides, in part, a novel surface coated carbon/carbon composite or graphite-like material, which is resistant to oxidation at high temperatures comprising: (A) a first coating of silicon and/or silicon carbide, and (B) a second coating comprising a material containing phosphorus chemically bound to oxygen and said oxygen is chemically bound to silicon.
Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.